1. Field of the Invention
This invention relates to the measurement and control of consistency in an aqueous flow stream, as it leaves a stage (i.e. Bleaching Stage) of the papermaking process. More particularly, the present invention relates to the measurement used for control of wood fiber and suspended solids in a papermaking pulp stock flow stream.
2. Description of the Related Art
In the paper industry, paper is made from wood fibers and chemicals and they are transported from one part of the paper mill to the other in a liquid medium that consists primarily of water once in this liquid medium, the fibers can be treated with chemicals to change their properties and improve paper quality. For example, chemicals can be added to bleach the fibers white or to cause the fibers to acquire certain mechanical properties.
In order for the paper making process to operate efficiently, the ratio of the weight of solids (fibers and other suspended solids) to the total weight of the mixture (fibers and solids and liquids) . . . termed "consistency" must be measured and controlled. The term "suspended solids" is anything in a mixture that will remain after all the liquids have been drained and/or evaporated away. Wood fiber is in fact a suspended solid, but since it is the main ingredient used to make paper, we refer to it separately.
The mathematical equation for consistency is according to Equation 1: ##EQU1##
The accuracy of a consistency meter is checked by extracting a representative sample of the mixture the meter is measuring and applying the above Equation 1, using the weight of the sample mixture and the weight of what is left after all moisture has been removed and comparing the results to what the meter indicated the consistency was at the time the sample was extracted.
Consistency of pulp stock is typically controlled by diluting it with water as the stock is pumped from a storage tank to the next stage in the process. It is standard practice to introduce the dilution water just ahead of the stock pump to obtain good mixing. Obviously, the consistency of the stock leaving the storage tank must be higher than what is needed for the next stage of the process in order for this to work.
As shown in prior art FIG. 1, the most basic control strategy utilizes a single consistency meter 16 at the end of the process stage 12 and a consistency controller 10 to manipulate a dilution water valve 6 to maintain the desired consistency. As can be seen in FIG. 1, consistency control system 2 includes papermaking storage tank 4, dilution valve 6, stock pump 8, consistency controller 10, process stage 12, process additive lines 14 and consistency meter 16. Process stage 12 may, for example, be a bleaching stage with additive lines 14 being bleaching chemical lines.
A more involved control strategy, as shown in prior art FIG. 2, includes a dilution water flow meter 22 and flow controller 24 for dilution water flow control that receives its' flow setting (flow setpoint) from consistency controller 10. This strategy accounts for fluctuations in the dilution water supply pressure. As can be seen in FIG. 2, consistency control system 20 includes storage tank 4, dilution valve 6, stock pump 8, consistency controller 10, process stage 12, process additive lines 14, consistency meter 16, dilution flow meter 22 and dilution flow controller 24.
As shown in prior art FIG. 3, even more involved control strategies have been used to bias (add or subtract a value to/from) the consistency controller's output 32 as additional liquids and/or suspended solids are introduced into the process so that dilution flow is adjusted accordingly. As can be seen in FIG. 3, consistency control system 30 includes storage tank 4, dilution valve 6, stock pump 8, consistency controller 10, process stage 12, process additive lines 14, consistency meter 16, dilution flow meter 22, dilution flow controller 24, and dilution setpoint 32.
It is also known that deadtime (measurement delay) is the biggest contributor to errors in process control, so it is important that the major sources of deadtime be identified and their contribution to total deadtime be reduced if possible. The most obvious and most significant cause of deadtime in the control of papermaking stock consistency results from the placement of the consistency measuring device (meter) with respect to the dilution water injection point, and until the present invention, significant deadtime was sometimes unavoidable.
For example, for processes requiring the introduction of liquids and/or suspended solids after the injection of dilution water, the consistency meter must be placed after the last addition point since that is where consistency ceases to change. This forces a significant delay (deadtime) between the time dilution water is injected and the resulting change in consistency is detected. The most advanced prior art control strategy outlined in FIG. 3 eliminates transport delays that would otherwise occur between the process additives and the consistency meter 22, but a significant transport delay still exists between the dilution water injection point and consistency meter 16.
There are many consistency meters on the market and the present invention is not limited to the use of any particular type. Instead, the present invention has to do with where to place a consistency meter in the papermaking process and how the virtual consistency meter's measurement is obtained. A virtual consistency meter is not a physical meter (one you can put your hands on), but instead a calculated consistency value that represents the consistency at the end of a particular stage in the paper making process after additional liquids and/or suspended solids have been introduced.
It is apparent from the above that there exists a need in the art for a consistency control system which can provide the control of consistency leaving a papermaking process stage with minimum deadtime (measurement delay). It is a purpose of this invention to fulfill these and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.